An air pollutant is defined as a compound added directly or indirectly by humans to the atmosphere in such quantities as to affect humans, animals, vegetation, or materials adversely. Air pollution requires a very flexible definition that permits continuous change. When the first air pollution laws were established in England in the fourteenth century, air pollutants were limited to compounds that could be seen or smelled – a far cry from the extensive list of harmful subtances known today. As technology has developed and knowledge of the health aspects of various chemicals has increased, the list of air pollutants has lengthened. In the future, even water vapor might be considered an air pollutant under certain conditions.
Many of the more important air pollutants, such as sulfur oxides, carbon monoxide, and notrigen oxides, are found in nature. As the Earth developed, the concentration of these pollutants was altered by various chemical reactions; they became components in biogeochemical cycles. These serve as an air purification scheme by allowing the compounds to move from the air to the water or soil. On a global basis, nature’s output of these compounds dwarfs that resulting from human activities.
However, human production usually occurs in a localized area, such as as city. In such a region, human output may be dominant and may temporarily overload the natural purification scheme of the cycles. The result is an increased concentration of noxious chemicals in the air. The concentrations at which the adverse effects appear will be greater than the concentrations that the pollutants would have in the absence of human activities. The actual concentration need not be large for a subtance to be a pollutant; in fact, the numerial value tells us little until we know how much of an increase this represents over ther concentration that would occur naturally in the area. For example, sulfur dioxide has detectable health effects at 0.08 parts per million (ppm), which is about 400 times its natural level. Carbon monoxide, however, has a natural level of 0.1 ppm and is not usually a pollutant until its level reaches about 15 ppm.
According to the passage, human-generated air pollution in localized regions ________.
A. can be dawrfed by nature’s output of pollutants
B. can overwhelm the natural system that removes pollutants
C. will damage areas outside of the localized regions
D. will react harmfully with natural pollutants
Đáp án B.
Key words: human-generated, localized regions.
Câu hỏi: Theo bài khóa, đâu là ý đúng về các chất ô nhiễm không khí do con người gây ra ở các vùng chuyên môn hóa?
Clue: “In such a region, human output may be dominant and may temporarily overload the natural purification scheme of the cycles”: Ở những vùng như vậy, chất thải của con người quá nhiều và có thể tạm thời quá tải áp đảo chương trình thanh lọc tự nhiên của chu trình.
Phân tích: Chu trình được nói đến ở trên là chu trình thanh lọc nhằm loại bỏ khí ô nhiễm. Do đó, ta chọn đáp án B. can overwhelm the natural system that removes pollutants: có thể áp đảo hệ thống tự nhiên loại bỏ các chất gây ô nhiễm. Các đáp án khác không phù hợp:
A. can be dwarfed by nature’s output of pollutants: có thể bị làm ít đi bởi những chất gây ô nhiễm ngoài tự nhiên.
C. will damage areas outside of the localized regions: sẽ gây hại khu vực bên ngoài một nơi nhất định.
D. will react harmfully with natural pollutants: sẽ phản ứng gây hại đến những chất ô nhiễm tự nhiên.